The number and variety of portable and mobile devices in use have exploded in the last decade. For example, the use of mobile phones, tablets, media players etc. has become ubiquitous. Such devices are generally powered by internal batteries and the typical use scenario often requires recharging of batteries or direct wired powering of the device from an external power supply.
Most present day systems require a wiring and/or explicit electrical contacts to be powered from an external power supply. However, this tends to be impractical and requires the user to physically insert connectors or otherwise establish a physical electrical contact. It also tends to be inconvenient to the user by introducing lengths of wire. Typically, power requirements also differ significantly, and currently most devices are provided with their own dedicated power supply resulting in a typical user having a large number of different power supplies with each being dedicated to a specific device. Although, the use of internal batteries may avoid the need for a wired connection to a power supply during use, this only provides a partial solution as the batteries will need recharging (or replacing which is expensive). The use of batteries may also add substantially to the weight and potentially cost and size of the devices.
In order to provide a significantly improved user experience, it has been proposed to use a wireless power supply wherein power is inductively transferred from a transmitter coil in a power transmitter device to a receiver coil in the individual devices.
Power transmission via magnetic induction is a well-known concept, mostly applied in transformers, having a tight coupling between primary transmitter coil and a secondary receiver coil. By separating the primary transmitter coil and the secondary receiver coil between two devices, wireless power transfer between these becomes possible based on the principle of a loosely coupled transformer.
Such an arrangement allows a wireless power transfer to the device without requiring any wires or physical electrical connections to be made. Indeed, it may simply allow a device to be placed adjacent to or on top of the transmitter coil in order to be recharged or powered externally. For example, power transmitter devices may be arranged with a horizontal surface on which a device can simply be placed in order to be powered.
Furthermore, such wireless power transfer arrangements may advantageously be designed such that the power transmitter device can be used with a range of power receiver devices. In particular, a wireless power transfer standard known as the Qi standard has been defined and is currently being developed further. This standard allows power transmitter devices that meet the Qi standard to be used with power receiver devices that also meet the Qi standard without these having to be from the same manufacturer or having to be dedicated to each other. The Qi standard further includes some functionality for allowing the operation to be adapted to the specific power receiver device (e.g. dependent on the specific power drain).
The Qi standard is developed by the Wireless Power Consortium and more information can e.g. be found on their website: http://www.wirelesspowerconsortium.com/index.html, where in particular the defined Standards documents can be found.
The Qi wireless power standard describes that a power transmitter must be able to provide a guaranteed power to the power receiver. The specific power level needed depends on the design of the power receiver. In order to specify the guaranteed power, a set of test power receivers and load conditions are defined which describe the guaranteed power level for each of the conditions.
Qi originally defined a wireless power transfer for low power devices considered to be devices having a power drain of less than 5 W. Systems that fall within the scope of this standard use inductive coupling between two planar coils to transfer power from the power transmitter to the power receiver. The distance between the two coils is typically 5 mm. It is possible to extend that range to at least 40 mm.
However, work is ongoing to increase the available power, and in particular the standard is being extended to mid-power devices being devices having a power drain of more than 5 W.
The Qi standard defines a variety of technical requirements, parameters and operating procedures that a compatible device must meet.
Communication
The Qi standard supports communication from the power receiver to the power transmitter thereby enabling the power receiver to provide information that may allow the power transmitter to adapt to the specific power receiver. In the current standard, a unidirectional communication link from the power receiver to the power transmitter has been defined and the approach is based on a philosophy of the power receiver being the controlling element. To prepare and control the power transfer between the power transmitter and the power receiver, the power receiver specifically communicates information to the power transmitter.
The unidirectional communication is achieved by the power receiver performing load modulation wherein a loading applied to the secondary receiver coil by the power receiver is varied to provide a modulation of the power signal. The resulting changes in the electrical characteristics (e.g. variations in the current draw) can be detected and decoded (demodulated) by the power transmitter.
Thus, at the physical layer, the communication channel from power receiver to the power transmitter uses the power signal as a data carrier. The power receiver modulates a load which is detected by a change in the amplitude and/or phase of the transmitter coil current or voltage. The data is formatted in bytes and packets.
More information can be found in chapter 6 of part 1 the Qi wireless power specification (version 1.0).
Although Qi uses a unidirectional communication link, it has been proposed to introduce communication from the power transmitter to the power receiver. However, such a bidirectional link is not trivial to include and is subject to a large number of difficulties and challenges. For example, the resulting system still needs to be backwards compatible and e.g. power transmitters and receivers that are not capable of bidirectional communication still need to be supported. Furthermore, the technical restrictions in terms of e.g. modulation options, power variations, transmission options etc. are very restrictive as they need to fit in with the existing parameters. It is also important that cost and complexity is kept low, and e.g. it is desirable that the requirement for additional hardware is minimized, that detection is easy and reliable, etc. It is also important that the communication from the power transmitter to the power receiver does not impact, degrade or interfere with the communication from the power receiver to the power transmitter. Furthermore, an all-important requirement is that the communication link does not unacceptably degrade the power transfer ability of the system.
Accordingly, many challenges and difficulties are associated with enhancing a power transfer system such as Qi to include bidirectional communication.
System Control
In order to control the wireless power transfer system, the Qi standard specifies a number of phases or modes that the system may be in at different times of the operation. More details can be found in chapter 5 of part 1 the Qi wireless power specification (version 1.0).
The system may be in the following phases:
Selection Phase
This phase is the typical phase when the system is not used, i.e. when there is no coupling between a power transmitter and a power receiver (i.e. no power receiver is positioned close to the power transmitter).
In the selection phase, the power transmitter may be in a standby mode but will sense to detect a possible presence of an object. Similarly, the receiver will wait for the presence of a power signal.
Ping Phase
If the transmitter detects the possible presence of an object, e.g. due to a capacitance change, the system proceeds to the ping phase in which the power transmitter (at least intermittently) provides a power signal. This power signal is detected by the power receiver which proceeds to send an initial package to the power transmitter. Specifically, if a power receiver is present on the interface of the power transmitter, the power receiver communicates an initial signal strength packet to the power transmitter. The signal strength packet provides an indication of the degree of coupling between the power transmitter coil and the power receiver coil. The signal strength packet is detected by the power transmitter.
Identification & Configuration Phase
The power transmitter and power receiver then proceeds to the identification and configuration phase wherein the power receiver communicates at least an identifier and a required power. The information is communicated in multiple data packets by load modulation. The power transmitter maintains a constant power signal during the identification and configuration phase in order to allow the load modulation to be detected. Specifically, the power transmitter provides a power signal with constant amplitude, frequency and phase for this purpose (except from the change caused by load-modulation).
In preparation of the actual power transfer, the power receiver can apply the received signal to power up its electronics but it keeps its output load disconnected. The power receiver communicates packets to the power transmitter. These packets include mandatory messages, such as the identification and configuration packet, or may include some defined optional messages, such as an extended identification packet or power hold-off packet.
The power transmitter proceeds to configure the power signal in accordance with the information received from the power receiver.
Power Transfer Phase
The system then proceeds to the power transfer phase in which the power transmitter provides the required power signal and the power receiver connects the output load to supply it with the received power.
During this phase, the power receiver monitors the output load conditions, and specifically it measures the control error between the actual value and the desired value of a certain operating point. It communicates these control errors in control error messages to the power transmitter with a minimum rate of e.g. every 250 msec. This provides an indication of the continued presence of the power receiver to the power transmitter. In addition the control error messages are used to implement a closed loop power control where the power transmitter adapts the power signal to minimize the reported error. Specifically, if the actual value of the operating point equals the desired value, the power receiver communicates a control error with a value of zero resulting in no change in the power signal. In case the power receiver communicates a control error different from zero, the power transmitter will adjust the power signal accordingly.
The system allows for an efficient setup and operation of the power transfer. However, there are scenarios where the power transfer system does not operate optimally.
For example, in the existing system, the power transmitter will enter the ping phase from the selection phase when it is detected that a new power receiver is introduced. However, if a power receiver device is e.g. permanently placed on the power transmitter, there is no initiating event, and the power receiver may remain in the selection phase and not be able to re-enter the power transfer phase. This may be a problem for devices that need repowering at intervals. For example, a battery powered device may permanently be placed on a power transmitter. After an initial charging of the battery when the battery powered device is first put on the power transmitter, the system will enter the selection phase. The device may be used while on the power transmitter and the battery may be discharged. At some stage, it may be required that the battery is recharged. However, as the system is in the selection phase it will not be able to perform such a recharging.
In order to avoid such scenarios, it has been proposed for the power transmitter to very occasionally enter the ping phase where it pings the power receiver to see if a new power transfer phase should be re-initiated. However, this is expected to be performed at an interval of several minutes which is too slow for many applications. Reducing the time between the pings will increase power consumption for both power transmitter and power receiver. Thus, reducing the time interval between pings to a value that is suitable for the most critical device/application would result in a large overhead and increased resource consumption which is completely unnecessary for the vast majority of devices.
In order to address this, it has been proposed that the system may leave the selection phase and initiate a new power transfer setup operation in response to receiving an active request from the power receiver. However, this requires that the power receiver can communicate an active message (i.e. it cannot use load modulation as there is no power signal provided by the transmitter). Such an active initiation by the power receiver may be advantageous but requires that the power receiver has sufficient stored energy to generate the message. However, this requires that recharging of the devices, and thus the devices cannot continuously remain in the selection phase.
Specifically, it has been proposed that a power receiver can wake-up a power transmitter by applying an active signal. The power receiver uses an energy source (e.g. a battery) available in the power receiver to generate the wake-up signal. However, not all devices contain a suitable energy source. Furthermore, if an energy storage, like a battery or a capacitor is present, this may become discharged e.g. after intensive use of an application or after a considerable amount of time during which a leakage or standby current has drained the available stored energy. Therefore, recharging will be required.
More generally, whereas the conventional approach may provide very suitable approaches for powering or charging a new power receiver when this is introduced, it tends to be relatively inflexible and not cater for all scenarios in which a power receiver may want to extract power from a power transmitter. Specifically, it merely allows the power receivers either to be powered by the power transmitter as part of a standard power transfer phase or to not be powered. However, many devices have different requirements at different times and furthermore these requirements can vary significantly between devices.
Hence, an improved power transfer system would be advantageous and in particular a system allowing increased flexibility, backwards compatibility, facilitated implementation, improved adaptation to varying power requirements, and/or improved performance would be advantageous.